It has been found that NMR spectroscopy is capable of producing cross-sectional images of organic bodies and is of particular value in the medical examination of the human body. Experiments in this field have been conducted previously and devices suitable for the stated purpose have been developed (Nachr. Chem. Tec. Lab. 28 (1980), No. 12, p. 860-865). This method of producing cross-sectional body images is generally referred to as NMR tomography, occasionally also as NMR Zeugmatography or NMR Imaging.
A considerable problem in NMR tomography is related to the production of a homogeneous magnetic field of the dimensions required to accommodate the section of a body being examined. In view of an expenditure that can be reasonably justified, the production of homogeneous magnetic fields of a size necessary for the examination in psrticular of the human body may probably have to be confined to the interior of current carrying coils because permanent electromagnets with pole pieces of sufficient size separated by an appropriately large distance would be too elaborate and especially much too heavy. Thus, the reference cited in the foregoing utilizes an electromagnet for NMR tomography which comprises four water cooled current carrying coils arranged in what is known as a Helmholtz Double Coil assembly.
To create homogeneous magnetic fields by means of such constituent coil assemblies necessitates an extremely accurate dimensioning and setting of the constituent coils to meet the high requirements as to homogeneity of the magnetic field and eliminate by way of compensation any interfering field components of a higher order. Even minor positional changes may adversely affect any such compensation and, consequently, the homogeneity of the magnetic field. Positional changes may easily occur due to different heat expansion of the elements forming the supporting structure for the coils. For this reason, when such compound coils are put to use in laboratories etc., care is being taken to ensure a constant room temperature. Any disturbing influences are, of course, the greater, the larger the coils are. In NMR tomography, extremely large coil arrangements are required in the examination of an organic body, be it human or animal, which must fit into the space encompassed by the coils. The minimum interior diameter of a coil to be used in the examination of the human body is approximately 650 mm.
A further source of error is the effect of external magnetic fields. While it is possible in principle to shield the rooms housing NMR apparatus from external magnetic fields, such protective measures involve a high degree of technical expenditure.
Another problem connected with NMR tomography is that the coils required to produce the high frequency magnetic field must likewise be of a large diameter. This has the effect that the coils act to a considerable degree as transmitting antennas, giving rise to substantial disturbances, especially, if standing waves are developing in the work room which may cause great localized fluctuations in the field strength of the HF field. Moreover, there is an appreciable loss of energy, and the sensitivity of the apparatus is also affected.
In addition to the adverse effect on the NMR tomography, radiation from a high frequency field may also affect electromagnetic devices stationed in the same room with the coils. This is true especially of peripheral devices required for use in conjunction with NMR tomography such as, for example, memory discs, floppy discs, etc.
In the prior art electromagnets, the current load on the coils and thus the power requirements are extremely high. Cooling means are usually necessary of a magnitude that not only involves substantial expenditures, but which also may be the cause for the creation of temperature gradients. These, in turn, may lead to varying heat expansion values, so that changes in the geometric arrangement of the structural elements may be necessary which are not to the best in an electromagnet of this type and, as previously indicated, may result in disturbances in the homogeneity of the magnetic field.
All of these problems appear to make it nearly impossible to produce an electromagnet which is insensitive to disturbances to the extent that it can be successfully used at the office of a radiologist, in diagnostic departments of hospitals, and in other zoological or biological institutions, and which at the same time is of a structurally simple design and easy to manufacture, with sufficiently small dimensions and low energy requirements to be suitable for practical application in the field.
The present invention has as its object to create an electromagnet of the afore-mentioned type.